1. Field of the Invention
The present invention relates to an image processing method. More particularly, the present invention relates to an image processing method that can, even though performing an error diffusion process, print an image in which a pattern or sweeping phenomenon specific to the error diffusion process does not appear.
2. Description of the Related Art
In a printing apparatus that prints an image by printing or non-printing of a dot, a quantization process that converts multivalued density data to binary data indicating printing (1) or non-printing (0) is required. Also, as such a quantization process, an error diffusion process has often been used.
FIG. 1 is a block diagram for explaining a configuration of a typical error diffusion process. Inputted multivalued image data (In) is added with a diffusion error (dIn) from a peripheral pixel by an adder 101 to become an input correction value (In+dIn). Then, the input correction value (In+dIn) is compared with a predetermined threshold value by a comparator 102 to output a value that is binarized into any one of printing and non-printing. For example, in the case where the inputted multivalued image data (In) is a value having 0 to 255, and the threshold value is 128, an output value (Out) of a pixel meeting [input correction value (In+dIn)≧128] is 255 (printing). On the other hand, an output value (Out) of a pixel meeting [input correction value (In+dIn)<128] is 0 (non-printing). At this time, the comparator calculates a difference (error) dOut=(In+dIn)−128 between the input correction value and the output value, and while applying predetermined weighting, stores the resultant in an error buffer 103. The error stored in this manner is, when an unprocessed pixel positioned around is subjected to the binarization process, used as the above diffusion error (dIn).
As described, the error diffusion process compares inputted multivalued image data with a predetermined threshold value to determine printing or non-printing, and is characterized by distributing (diffusing) an error occurring at the time of the determination to an unprocessed pixel positioned around.
However, in a simple error diffusion method as illustrated in FIG. 1, due to a constant threshold value, a specific pattern or sweeping phenomenon may be recognized in a binarized image. Regarding this, for example, Japanese Patent Laid-Open No. 2001-333277 discloses a method that adds random number components to a fixed threshold value to prepare a plurality of different threshold values as a threshold value matrix as illustrated in FIG. 2, and repetitively uses the matrix. Also, Japanese Patent Laid-Open No. 2004-120133 discloses a method that uses a threshold value matrix prepared as in Japanese Patent Laid-Open No. 2001-333277 in not the simple repetitive manner but a rotation manner.
However, in the method disclosed in Japanese Patent Laid-Open No. 2001-333277, the prepared threshold value matrix is repetitively used, and therefore, in an output image, a periodic pattern depending on a size of the threshold value matrix is sometimes recognized. In order to obscure such a periodic pattern, it is effective to increase a size of the threshold value matrix; however, in this case, there occurs a problem of increasing a memory capacity for storing the threshold value matrix.
On the other hand, according to the method disclosed in Japanese Patent Laid-Open No. 2004-120133, the threshold value matrix is not used in the simple repetitive manner, and therefore even in the case of using a relatively small threshold value matrix, a period of the threshold value matrix is unlikely to appear in an image. Accordingly, without influencing a memory capacity, the same effect as that for the case of an increased matrix size can be obtained. Also, even though the same threshold value matrix is used for a plurality of color inks, by changing an amount or direction of the rotation for each of the colors, periodic unevenness in a color image can also be avoided.
However, in the method disclosed in Japanese Patent Laid-Open No. 2004-120133, due to the rotation of the threshold value matrix, the number of times to read the threshold value matrix is increased to increase a load on a rotation process. As a result, image processing speed is reduced to cause a reduction in printing speed of a printing apparatus.
Also, if a multipass printing method is employed for an image that is quantized with use of a threshold value matrix as described above, due to synchronization between a mask pattern used for multipass printing and the threshold value matrix, the occurrence of periodic sparseness and denseness of dots is also sometimes recognized. In this case, even if the multipass printing is performed, sometimes, an effect of the multipass printing is unlikely to appear, or slight displacement of a printed position at the time of the multipass printing makes the above sparseness and denseness significant to cause image deterioration.